Astronomers have discovered about 2000 planets around other stars, but they have few hard facts about what they are like, such as the contents of their atmospheres. Have they clear skies like Star Wars’ Tatooine or are they overcast like the planet home of Cloud City? Now, a team of astronomers using the Hubble and Spitzer space telescopes have gathered enough data to compare 10 large exoplanets, finding a range of atmosphere types, and to propose a solution to an early mystery of exoplanet atmospheres: why some don’t seem to have enough water.
It’s hard to detect—let alone study—exoplanets because they are very far, very faint, and are close to a source that is about a billion times brighter: their star. The vast majority of exoplanets were discovered by observing their transits: If their orbits are edge-on, viewed from Earth, then they dim their star when they pass in front. Transits can also reveal the size of an exoplanet and, sometimes, what’s in its atmosphere. That’s because when the planet is in front of the star, some of the starlight passes through its atmosphere and is scattered or absorbed by gases and molecules in it, leaving a spectral fingerprint in the light that eventually reaches Earth.
But that fingerprint is so faint that until a few years ago, only a couple exoplanets had given up any atmospheric information. And these two—known as HD 209458b and HD 189733b—had a signature for water that was much fainter than astronomers had been expecting from planet models and the composition of their stars. The explanation boiled down to a “nature versus nurture question,” says Nikku Madhusudhan of the University of Cambridge in the United Kingdom. Either the planets were born with less water, through some as-yet-unknown process, or they’ve got the water and are hiding it, perhaps under clouds or haze in the atmosphere. (Even if the clouds are made of water, they are still hiding much of the atmosphere, so any water signature will be reduced by the presence of clouds.)
Prospects for studying exoplanet atmospheres brightened in 2009 when the instruments on Hubble were upgraded, enabling astronomers to get more detailed spectra from stars. Suddenly, there were about a dozen exoplanets that might give up their secrets. They are all called hot Jupiters, large gaseous planets in orbits hugging their stars and with bulging atmospheres to leave a clear fingerprint through scattering and absorption of nearby starlight.
A team led by David Sing of the University of Exeter in the United Kingdom has carried out detailed studies at optical and infrared wavelengths on eight of those new exoplanets, adding to the data already obtained on the original two. As they report online today in Nature, the distant planets clearly contain water and some are cloudy. Moreover, the strength of the water signature depends on the amount of clouds, suggesting that the hazy planets may be concealing water. Others welcome the study. “It’s a beautiful set of data. Comparative planetology has taken a big step forward,” says Sara Seager of the Massachusetts Institute of Technology in Cambridge. And “it’s cool to see the diversity of exoplanets,” she says.
But it remains hard to draw strong conclusions about the nature of exoplanet atmospheres. Because this study simply compared cloud cover with the water spectral feature, “you cannot make a robust claim,” Madhusudhan says. But he says the study is “a natural first step in the right direction.”
Although the field is struggling to get higher quality data now, things will change after the 2018 launch of the James Webb Space Telescope (JWST), a successor to Hubble that will detect infrared light. “JWST will give us a complete look at atmospheres, right out to the far infrared where the real chemistry happens,” Sing says.